Eight glasses a day is a number without a source, and I distrust numbers without sources. Nobody can point to the study, and the figure ignores body weight, exercise, and whether it is July. Some people need two litres, some need four, and one number for everyone is the same mistake as one shoe size.
The Water Intake Calculator starts from a guideline that does have a source, 30 to 40 ml per kilogram of body weight, takes the middle at 35 ml, and adds for exercise and hot weather. Take that as the starting figure. Then adjust on evidence: urine colour, which is the one measurement you get for free every day.
The target, and what moves it
The US National Academies put adequate total water intake at about 3.7 litres a day for men and 2.7 for women, from all sources. Roughly a fifth of that arrives in food, so the drinking target is nearer 3.0 and 2.2 litres. Those are population averages, and the per-kilo rule gets closer to you: 70 kg times 35 ml is 2.45 litres, 90 kg is 3.15.
The calculator adds 350 ml for every 30 minutes of moderate exercise, and a flat 500 ml on a hot or humid day. Other things that push the need up, and that no calculator asks about:
- Altitude, through faster breathing
- Fever, vomiting, or diarrhoea
- Pregnancy, about 300 ml more, and breastfeeding, about 700 ml more
- A diet high in protein or fibre
- Alcohol, which is a diuretic at any real quantity. Coffee is not, in normal amounts; see the myths below.
The body gives three readings: thirst, urine colour, and energy. Pale yellow urine is the target. Dark means drink. Clear, all day, means you are past the target and can ease off.
A tracking method that survives week two
Tracking water does not need an app, and the methods that last are the ones that need nothing opened.
The bottle. One bottle of a known size, 750 ml or a litre, and a target in bottles. Two refills is two litres. It is the method I would pick, because the count is visible on the desk and there is nothing to log.
The clock. A glass every waking hour. Sixteen hours at 250 ml is four litres, which is more than most people need, so every other hour is closer for a desk day. A Countdown Timer set to 60 minutes does the reminding for the first two weeks. After that the habit does it.
The meals. One glass on waking, one before and one with each meal, one before bed. Eight glasses, no counting, and the anchors already exist.
Apps and smart bottles. WaterMinder, Hydro Coach, and Plant Nanny make it a game, and games work until they do not; most people stop opening them within a month. Smart bottles such as HidrateSpark log the sips themselves for around €50 to €70. I have not used one, so no opinion beyond the price.
Whatever the method, measure the result, not the effort. The result is the colour.
Tracking water does not need an app, and the methods that last are the ones that need nothing opened.
Signs worth acting on
Mild dehydration, one to two percent of body water, produces symptoms that are easy to blame on something else.
- Dark urine. The most reliable sign and the first to check.
- A headache in the afternoon. Before the painkiller, a glass of water and twenty minutes. If it goes, you have your answer.
- The afternoon slump. Fatigue and poor concentration arrive early in dehydration. Water is a cheaper test than another coffee.
- Dry mouth and lips that lip balm does not fix.
- Fewer than four trips to the toilet in a waking day.
- Muscle cramps during or after exercise, from fluid and electrolyte loss together.
Severe dehydration, from about three percent loss, looks different: racing heart, dizziness, confusion, fainting. That is medical attention, not a bigger bottle.
All of these overlap with other conditions. If two weeks of drinking to target do not shift the headaches or the fatigue, see a doctor, because water was not the cause.
Thirst impersonates hunger. If you are tracking food as well, the Calorie Calculator gives the intake target, and a glass of water before the snack tells you whether the snack was needed.
The myths I would stop repeating
Coffee dehydrates you. Not at normal amounts. Caffeine is a mild diuretic, and the water in the cup more than covers it. Moderate coffee counts towards the day's intake. At five or more cups the net effect may tip slightly the other way.
Eight glasses a day. No study, no source. Use the per-kilo figure and your own signals.
Clear urine is the goal. Consistently clear means you are drinking more than you need. The goal is pale yellow. Far past that, blood sodium can drop (hyponatremia), which is dangerous in extreme cases.
Thirst means you are already dehydrated. By the time you feel it you are one to two percent down, which is mild, and it is the body's regulation working as designed. Drink when thirsty and you will be fine on an ordinary day. Drink ahead of it before exercise and in heat.
Water flushes toxins. The kidneys filter the blood at the rate they filter it. Enough water lets them work; extra water does not make them work harder.
Sparkling water does not count. It counts. Carbonation changes nothing about absorption. If it bloats you, drink still.
**Coffee dehydrates you.** Not at normal amounts.
FAQ
Can I drink too much water?
Yes. Hyponatremia is low blood sodium from drinking far more than the kidneys can pass, which is about a litre an hour. It is rare in daily life and a real risk in marathons and ultras where people drink to a schedule without replacing salt. Stay under a litre an hour and you are safe.
Do tea, coffee, juice, and soup count?
Yes, all of them, and milk. Alcohol is the exception. Sugary drinks hydrate, and carry everything else they carry.
Water with meals: before, during, or after?
Any of them. The idea that water dilutes digestive enzymes is not supported. A glass before a meal makes some people eat a little less, which may or may not be what you want.
How much more for exercise?
The calculator adds 350 ml per 30 minutes of moderate effort; add more for hard sessions and heat. The precise method is a scale: weigh yourself before and after, and every half kilo lost is about half a litre of fluid. Replace about one and a half times what you lost over the next few hours.
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